When a circuit breaker containing sulphur hexafluoride at a pressure of several bars is placed in a very low ambient temperature (e.g. less than -20.degree. C.), a portion of the dielectric SF.sub.6 gas condenses. This reduces the apparatus' circuit-breaking capacity and increases the risk of internal arcing.
One known way of solving these drawbacks is to provide such a circuit breaker with means for heating the gas in its current-interrupting chamber.
Such means are effective only if considerable heating power is used, and in any event cannot prevent the risk of SF.sub.6 condensing on the porcelain walls of the circuit interrupting chamber and its support, and it is such condensation which gives rise to the above-mentioned arcing.
Preferred embodiments of the invention provide a circuit-breaker requiring such heating at very low power only.
This is done using a circuit breaker of the type described in French Pat. No. 85 074 37 filed May 15, 1985 (published under the number 2 582 145 and equivalent to U.S. patent application Ser. No. 862 941 filed May 14th, 1986), which includes a condenser outside the envelope and at ground potential, said condenser being associated with means for heating the liquefied SF.sub.6 in the condenser.